This invention relates to the polymerization of olefins and particularly relates to the preparation of high molecular weight, low crystallinity polyolefins using a Ziegler-Natta catalyst system.
As is well known in the art, effective catalyst systems for polymerization of 1-olefins are Ziegler-Natta catalyst systems, such as those obtained by combining a transition metal compound of group IVB of the Periodic Table with an activator which is an organometallic compound of Group IIIA of the Table. Catalyst systems containing TiCl.sub.4 or TiCl.sub.3 and aluminum alkyl or aluminum alkyl chloride activators are known to provide mixtures of crystalline and low crystallinity polypropylene. However, low crystallinity polyolefins having levels of crystallinity on the order of only 10 to 20% are difficult to obtain using these catalysts, and high levels of catalysts are required to obtain satisfactory yields of polymer, thus necessitating special techniques in recovering the product to remove residual catalyst.
The use of TiCl.sub.3 and TiCl.sub.4 -treated magnesium chloride supported catalyst components or hydrocarbon-insoluble reaction products of a tetravalent halogenated titanium compound and a magnesium alcoholate with organoaluminum compounds to polymerize 1-olefins is also disclosed in the art. Supported catalyst components of these types generally provide polyolefins having high crystallinity, on the order of 65% or more.
Further, it is known from U.S. Pat. No. 3,933,934 to Bailly et al. that atactic waxes can be produced with a Ziegler-Natta catalyst system wherein the transition metal component is formed by the reaction of metallic magnesium, an alkyl halide and a titanium compound and the activator is an organoaluminum compound. The magnesium-based titanium compound used by Bailly et al. as the transition metal component of the catalyst is a hydrocarbon insoluble material which must be separately prepared and isolated before use in the polymerization. Additionally, due to the insoluble nature of the catalyst component, some residue accumulates in the resulting polymer, thus necessitating costly and labor intensive clean-up procedures to obtain polymers of good clarity, color and stability.
Recently catalyst systems which are soluble in the reaction medium have been described in the art. For example, van den Berg in U.S. Pat. No. 4,319,010 teaches using a catalyst obtained by mixing a titanium compound, a soluble magnesium salt or complex and an organoaluminum halide in the reaction solvent to obtain high yields of high molecular weight crystalline homopolymers of ethylene at low catalyst levels. See also British publication No. 2,039,501 A which teaches using a catalyst system containing a magnesium halide which has been solubilized with an electron donor, a transition metal compound and an organoaluminum compound in the continuous polymerization of ethylene or mixtures of ethylene with small amounts of other alpha-olefins in hydrocarbon solvents.